Kinematic gait analysis is increasingly used for a wide variety of purposes, including clinical evaluation to aid surgical decision-making and the study of joint and limb motion for the design of prostheses and braces. Standard gait analysis procedures currently involve the estimation of skeletal motion by tracking the movement of targets placed on the skin. The motion of the skin and underlying soft tissues relative to the bone is known to be a source of error in the measurement of skeletal motion, but the magnitude and pattern of the soft tissue motion has not been reported in the literature. The purpose of this project is to determine the degree and pattern of error introduced into standard kinematic gait analysis procedures by soft tissue motion around the femur and shank. A critical initial phase of this project is the development of a relatively safe, simple, and reproducible method for attaching and tracking targets fixed to bone. Measurement of actual bone movement is accomplished using a percutaneous skeletal tracker (PST), a device designed to achieve skeletal fixation of targets. A PST is attached to the distal femur and distal shank; surface mounted targets are also attached using standard procedures. A three-dimensional target tracking system measures the movement of both target sets as subjects walk in the laboratory. The six degree-of-freedom motions of the shank and thigh are calculated using each target set, and the difference in motions represents the error introduced into the measurements by the soft tissues. Three subjects have undergone PST placement and data collection without complications. Rigid PST fixation to the distal shank was achieved for each subject. Fixation was not sufficient at the distal femur, however, but several important questions concerning pin placement at the knee were addressed. The skeletal motion data collected for the shank are currently being analyzed.